The present invention relates to a process control system with a means for supervising a variation of a process variable representing a state of a process to be controlled.
Conventional process control systems have used a process indicating meter in which process variables, representing a current state of a process which come from sensors provided at the respective portions of a system to be process-controlled, for the purpose of process supervising and the set-point values of the process variables, are indicated on a scale to check to see whether the process variables fall within a tolerable range. Another example uses a cathode ray tube (CRT), in which a simulative chart is depicted on the screen, with marks indicating a tolerable range. In another example for a number of control loops, a tolerable deviation range is covered with a green band and a red pointer is used for indicating a process variable. In this example, when the pointer moves out of the green band, this state is markedly observed. These process supervising methods, however, merely provide information whether a deviation amount of each process variable from its set-point value is within a tolerable range or not. For this reason, when a process variable, which is a controlled variable, is in the tolerable deviation range, an operator can not know whether the process variable will go out of the tolerable deviation range after a short time, similarly, when it is outside the tolerable deviation range, he can not know whether it will enter the tolerable range after a while. When positively knowing the trend of such a process variable, he must carefully observe the supervisory apparatus for each control loop. For this reason, the tolerable deviation range is set up strictly so that an excessive deviation of the process variable, which is more important, is passed over frequently.
Also in the process control, an indicating controller such as a PID indicating controller is provided and the process control is automatically carried out without manual operation. When the process control starts or stops, controlling instruments or the process are abnormal, or when it is desired to settle a process variable to a set-point value in the shortest time, an operator directly determines a controlling amount to control the process. Specifically, the operator performs the process control by determining the controlling amount while referring to the position or the movement of a pointer of the indicating controller, which indicates a process variable, or a trend record of the process variable by a trend recorder.
A deviation quantity between a process variable and its set-point value, which is one of the factors to determine the controlling amount in the process control is displayed on an indicator in the form of a bar length. Accordingly, the operator can read easily and accurately the process variable indicated by the indicator. In controlling the process in a transient condition, a deviation quantity and the differential value of the process variable representing a change state of the process, are needed for the factors to determine the controlling amount. By convention, the operator empirically determines the differential value from a motion of the pointer. For this, the change state of the process variable is poor in preciseness and accuracy. The control accuracy depends largely on individual operators and the controlling time differs for each operating even by the same operator. This encumbers a standardized and systemized process control, and necessitates a skill for the process control.
With an intention of solving this problem, a signal display unit was proposed in which a present value of the process variable and its differential value are displayed as phase points. In the display unit, however, a set-point value and a deviation value between the present value and the set-point value are not displayed, although the differential value is displayed. Therefore, an operator can not directly judge whether the process variable is approaching to or is departing from the set-point value. Accordingly, he often makes an error of the control direction. Moreover, the deviation quantity for determining the controlling amount is not indicated so that it is very difficult to set a process variable to a set-point value.
As described above, a serious problem of the conventional process control system is that, when a process variable is so controlled as to make it coincide with a set-point value, a change trend of process can not correctly be grasped.